It may be best to figure out a way to wire up your monitor multiple inputs up to the various desktops, and using the monitor input switch buttons. For the input, synergy (http://synergy-project.org/) which I haven't had use for in a long time. Otherwise, just have some close at hand usb hubs close and move the cables around. There exist 'cleaner' KVM devices to do this, but they are way expensive. If your monitor inputs are lacking, new monitors are likely cheaper than the KVM device you would need to not get new monitors. Monitors with three digital inputs would probably be the easiest thing to meet the requirements verbatim.
For another, I'm really wondering why you feel this need so strongly. What tasks are you spreading amongst all these systems? How many of these tasks *really* indicate need for directly using the attached 'head' versus remote access (RDP, VNC, ssh, whatever). Is there a good reason that the things that really need direct connectivity can't be grouped into a single system? If not a single system, narrow down to 2 PCs and comfortably fit on your monitor inputs.